The application relates generally to gas turbine engines, and more particularly, to engine case structures, such as mid turbine frame assemblies and similar structures.
A mid turbine frame (MTF) assembly, sometimes referred to as an interturbine frame, is located generally between a high turbine stage and a low pressure turbine stage of a gas turbine engine to support one or more bearings and to transfer bearing loads through to an outer engine case. The MTF assembly typically includes an inner case disposed around a rotor shaft of the gas turbine engine and the bearings supporting the rotor shaft. An outer case is disposed around the inner case to form a flowpath between the inner case and the outer case. Struts extend across the flowpath to connect the inner case to the outer case and to form a load path between the inner case and the outer case. The MTF assembly is thus a load bearing structure, and the safety of load transfer is one concern when a MTF assembly is designed.
In the past, some MTF assemblies have been formed by integrally forming the struts and inner case through a common casting and bolting the outer casing to the struts. In other previous MTF assembly designs, the struts have been mechanically connected to both the inner case and the outer case. In the MTF assembly designs where the struts are mechanically connected to both the inner case and the outer case, slots and grooves have been formed in both the outer case and the inner case to help prevent the struts and the mechanical connections of the struts from moving and coming loose. However, forming grooves and slots in the inner case and the outer case can create stress risers in the inner case and the outer case that can reduce the service life the MTF assembly. Accordingly, there is a need to provide improvements over previous MTF assemblies.